1. Technical Field
The present application relates generally to nanotube fabrics and methods of making same and, more specifically to carbon nanotube fabrics and methods of making same for use in electromagnetic radiation detecting and sensing systems.
2. Discussion of Related Art
Detectors are an integral part of optical circuits and components (emitters, modulators, repeaters, waveguides or fibers, reflectors, resonators, etc.) and are used for the sensing of electromagnetic (EM) radiation. Photoconducting materials, typically semiconductors, have electrical properties that vary when exposed to EM radiation (i.e. UV, visible and IR). One type of photoconductivity arises from the generation of mobile carriers (electrons or holes) during absorption of photons. For semiconducting materials, the absorption of a specific wavelength of light, hence photon energy, is directly proportional to the band gap of the material (Eg=h□=hc/□, where Eg is the materials band gap, h is Plank's constant (4.136×10−15 eVs), c is the speed of light in a vacuum (2.998×1010 cm/s) and □ is the wavelength of the radiation). If the band gap energy is measured in eV (electron Volts) and the wavelength in micrometers, the above equation reduces to Eg=1.24/□. A photodiode (i.e. p-n diode, p-i-n photodiode, avalanche photodiode, etc.) is the most commonly employed type of photoconductor as described in Kwok K. N G, “Complete Guide to Semiconductor Devices,” IEEE Press, John Wiley & Sons, 2002, pages 431-437. Light detection is ideally suited for direct band gap semiconductors such as Ge, GaAs, etc.; however, indirect band gap semiconductors (where an additional phonon energy is required to excite an electron from the valence band to the conduction band, making these detectors less efficient), such as Silicon, are also used as photodetectors. Probably the most widely known type of photodetctor is the solar cell, which uses a simple p-n diode or Schottky barrier to detect impinging photons. Besides silicon, most photodetectors do not integrate with current microelectronics technology, usually detect only a specific wavelength (i.e. 1.1 □m for Si, 0.87 □m for GaAs, 0.414 □m for □-SiC and 1.89 □m for Ge), and require multiple detectors to detect a broad band of wavelengths (hence photon energy).
There are several other techniques that can be employed to detect EM: by a change in resistance induced from a temperature rise in a detecting medium (bolometers), as described in Kwok K. N G, “Complete Guide to Semiconductor Devices,” IEEE Press, John Wiley & Sons, 2002, pages 532-533, or changes in a materials internal dipole moment when impinging radiation causes a temperature rise (pyroelectric detector). Bolometers can be constructed from either metallic, metallic-oxides or semiconducting materials. Since bolometers detect a broad range of radiation above a few microns, bolometers are typically cryogenically cooled to reduce the detection of blackbody radiation that is emitted from the detector material, which leads to a high background noise.
Detectors are critical for communication and EM sensing applications, specifically in the infrared (IR) spectrum, □>0.75 □m. Currently, many IR sensors are expensive, require advanced fabrication techniques that are not compatible with Si—CMOS technology, have limited resolution and do not operate at room temperature (hence require cooling). Conventional IR detectors mainly employ bolometers, more specifically microbolometers.
IR Focal Plane Arrays (IRFPAs), which operate in the 8-12 micron band, have experienced a revolution since the introduction of vanadium dioxide (VOx) microbolometer arrays. Until that time, tactical and strategic applications had relied upon HgCdTe IRFPAs, which operated at 77 K and necessitated the use of mechanical coolers. The HgCdTe system was plagued by defects, high processing costs, low yields and substantial weight (eliminating crucial man-portable applications). The introduction of VOx, serving as the TCR (Temperature Coefficient of Resistance) IR radiation sensing layer that is deposited on a thermally isolated cantilever beam, based microbolometers overcame many of the issues associated with HgCdTe. VOx technology can be fabricated with CMOS design and process technologies on silicon wafers, with a lower built in unit product cost. VOx devices also do not require cooling to 77 K—only thermal stabilization. This technology also enabled the introduction of man portable IR systems such as Thermal Weapon Sight and helmet integrated visible/Long-Wave IR (8-12 □m) fused imagery, which would not have been possible with cooled HgCdTe.
A major advantage of VOx technology is its CMOS compatibility. CMOS technology provides improved performance and increased resolution. Advanced CMOS processing techniques also enables the development of a reflective cavity coating that allows IR energy to reflect off the bottom of a cavity and to be absorbed by the VOx layer, increasing the efficiency of IR adsorption. Because of advanced CMOS process development and integration, detector unit cells have shrunk and the fill factors have increased, resulting in 640×480 pixel IRFPAs, thereby increasing the resolution of the cameras that these IRFPA are deployed in. The resolution of the IRFPA is directly related to the CMOS technology node and the minimum pixel size of the bolometer. For VOx microbolometers, increasingly higher resolution IRFPAs are not possible because of limits associated with the TCR of VOx: below 25 microns (required for 640×480 resolution), the TCR of VOx becomes non-linear. Additionally, improvements in analog preamp designs in CMOS have increased the signal to noise ratio for un-cooled IRFPAs.
Despite the improvements VOx has contributed to microbolometers, there is evidence that the improvement curve for this material is slowing down and some fundamental limits are preventing the use of these arrays in modern IR systems. First, there is a lack of responsivity in the vital Mid-Wave IR (MWIR) band (3-5 □m). A second limitation is that for UV applications, such as threat warning of missiles, VOx technology is not able to detect UV signatures from missile plumes. Although thermal isolation has improved to the levels of 20 mK, further improvements are required; therefore, another limitation of VOx based IRFPAs is that the performance Noise Equivalent Temperature Difference (NETD) is saturating, related to the limits of VOx absorption. The need for multi color (MW/LWIR) systems in tactical and strategic systems is also a necessity, which is not possible with VOx based microbolometers. Another issue with VOx is the inability to scale below 25 micron pixel sizes, limiting detectors to 640×480 pixels. As stated above, below 25 microns, the TCR of VOx becomes non-linear. Additionally, the 1/f noise of VOx increases as the pixel dimension decreases.
Carbon nanotubes (CNTs) are a promising material for electromagnetic (EM) detection and have recently been investigated for their unique optical properties, focusing on the emission and detection of IR radiation (see Sheng, et al., “Exciton dynamics in single-walled nanotubes: Transient photoinduced dichroism and polarized emission,” Physical Review B, 71 (2005), 125427, Perebeinos, et al., “Scaling of Excitons in Carbon Nanotubes,” Physical Review Letters, 92 (2004), 257402, Ugawa, et al., “Far-infrared to visible optical conductivity of single-wall carbon nanotubes,” Current Applied Physics, 1 (2001) 485-491, Lehman, et al., “Single-wall carbon nanotube coating on a pyroelectric detector,” Applied Optics, 44 (2005), 483-488, Itkis, et al., “Bolometric infrared photoresponse of suspended single-walled carbon nanotube film,” Science, 312 (2006), 413-416 and Mohite, et al., “Displacement current detection of photoconduction in carbon nanotubes,” Applied Physics Letters, 86 (2005), 061114).
Typical band-gaps for carbon nanotubes range from 0.6-1.2 eV, depending on the diameter of the CNT, where the band gap is inversely proportional to the diameter of the nanotube, which correlates to detection and emission of radiation in the IR spectrum. Several approaches to CNT light emission and detection have been investigated, namely the generation of excitons to produce a photocurrent or the generation of heat which changes the resistance of nanotube material (bolometer). Photoluminescence and photo-detection studies have demonstrated that carbon nanotubes, specifically single-walled nanotubes (SWNTs), absorb radiation at between 0.6-2 eV, correlating to the transition between van Hove peaks of the nanotubes (S11, S22 and M11). Carbon nanotubes have also demonstrated extremely high absorption coefficients of the order of 104-105 cm−1, higher than HgCdTe and much higher than VOx. Combining these properties allows for CNTs as a possible candidate for replacing HgCdTe and VOx for resistive microbolometer material.
A paper by Itkis demonstrated the creation a non-CMOS compatible CNT bolometer using a thick suspended mat of as-deposited, arc-grown SWNTs. Using a 0.94 □m emitting IR laser at 12 □W, the mat of CNTs demonstrated a sizable decrease in conductivity of the CNT mat with an S/N ratio of 100. TCR values measured by Itkis are comparable to vanadium dioxide, which strongly suggests the ability of the CNT bolometer to operate at room temperature. It was also noted that a decrease in mat thickness improves the sensitivity of the CNT bolometer; however, their fabrication technique can not produce reliable thickness and is not scalable to monolayered fabrics.
Despite the work performed by these groups there are still many concerns that need to be addressed before a reliable CNT-IR sensor becomes a reality. The first concern necessitates the need for processes that can be employed to fabricate the CNT-IR sensors with CMOS compatible technology and incorporated within silicon electronic devices. A possible disadvantage with CNTs is their varying band gaps; therefore, a functionalization scheme may need to be developed that produces CNTs with repeatable properties.